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#### Per Unit Check Box

The Per Unit check box on the Feature Control Frame tab in the XactMeasure GD&T dialog box (Insert | Dimension | <dimension>) appears for flatness and straightness dimensions. Use it to specify a flatness per unit area or a straightness per unit length.

When you mark the Per Unit check box, Planner changes the Feature Control Frame Editor area by adding an additional line where you can enter a tolerance value as well as the unit area or unit length values:

Feature Control Frame Editor (per unit flatness)

Feature Control Frame Editor (per unit straightness)

Planner now automatically uses a step increment that ensures that the "per unit window" includes maximum overlap as it "slides" over all the point data.

Per Unit Flatness Overlap Details:

The overlap between unit areas for flatness is usually 90%. This is the case if both of the following are true:

- TL <10 * UL

where TL is Total Length and UL is Unit Length.

- The number of points is less than 9,999.

The overlap decreases to a minimum of 50% if either of the following are true:

- TL > 25 * UL

where TL is Total Length and UL is Unit Length.

- The number of points is greater than 24,999

In table form, this is expressed as:

Overlap | Number of Points | Total Length Divided by Unit Length
90% | 5,000 | 5
80% | 10,000 | 10
70% | 15,000 | 15
60% | 20,000 | 20
50% | >= 25,000 | >= 25

Once you enter the desired information and create the Feature Control Frame (FCF), Planner also applies the flatness per unit area or straightness per unit length in the FCF:

For example, a flatness FCF with an overall flatness tolerance of .05 and a per unit tolerance of .01 per unit area of 10 x 10 would look like this:

Example of a Finished Per Unit Flatness FCF

For example, a straightness FCF with an overall straightness tolerance of .0254 with a per unit tolerance of .012 / per unit length of 10 would look like this:

Example of a Finished Per Unit Straightness FCF

Planner also supports the "square area" symbol that was added in the Y14.5 2009 standard. This symbol indicates when the area is exactly square, which means that the width and length values of the per unit area are identical. To use this symbol, select the <UA> field and choose the Square Area symbol from the list:

Example of Selecting the Square Area Symbol

The FCF appears as follows. The square area symbol indicates that the per unit area value following it has an identical width and length (in this case the 10 meaning a 10 X 10 area):

Example of a Finished Per Unit Flatness FCF with Square Area Symbol

The square area symbol also appears in your report output.